A Fiber Bragg Grating or FBG is a passive optical component obtained by imprinting a local, longitudinal and periodic modulation of the refractive index of the optical fiber core. In view of the local change of the refractive index, any light propagating along the fiber core undergoes partial reflection at each of the grating layers. As a consequence of the periodicity of the index modulation, constructive interference of the reflected light occurs for the wave vectors that meet the Bragg condition λB=2nΛ, where λB is the wavelength at the peak of the reflected light spectrum, or Bragg wavelength, Λ is the grating spatial periodicity, and n is the core refractive index. This implies that a portion of the incident spectrum is not transmitted, instead, it is reflected by the Bragg grating.
Since the wavelength reflected by the Bragg grating is a function of n and Λ, changes in temperature, ΔT, or in the strain to which the grating is submitted, Δε, cause significant changes in these parameters, this leading to changes in the Bragg wavelength that can be set forth as Δλ=λB(αΔT+βΔε).
Bragg grating sensors are employed for the measurement of several physical and chemical parameters. The mechanisms employed in Bragg grating sensing are usually based on the transference to the fiber of a strain, the origin of which can be an elastic structure such as a spring or a diaphragm. The principle of operation of the FBG sensors consists of monitoring the Bragg wavelength changes, Δλ, and of correlating them to the changes in the mensurand value.
Good measurement resolution can be obtained according to the wavelength measurement instrumentation. If a wavelength measurement system providing ±1 pm uncertainty is employed, the FBG technique can provide resolutions as high as tenths of Celsius degrees and few μm/m for temperature and strain changes, respectively.
Hydrogels are made up of a combination of a solid crosslinked polymeric chain and a neighboring aqueous solution. Whenever stimulated for example by changes in temperature, electric and magnetic fields and pH of the medium, these materials can undergo a pronounced and reversible change in their hydrodynamic volume. The processes responsible for hydrodynamic volume change are governed by interactions between the polymeric network and the aqueous solution. In pH-sensitive gels, the amount of hydrodynamic volume change is governed by the equilibrium between the restoring force provided by the crosslinks and the osmotic force caused by the diffusion of ionic species. High Young modulus values have been experimentally predicted and observed for various hydrogels, and a variety of chemical-mechanical actuators have been built. To this respect, see the articles by Brock, D., Lee, W. J., Segalman, D., Witkowski, M., “A Dynamic-Model of a Linear-Actuator Based on Polymer Hydrogel”, Journal of Intelligent Material Systems and Structures 5, 764 (1994); Shahinpoor, M., “Micro-electro-mechanics of Ionic Polymer Gels as Electrically Controllable Artificial Muscles”, Journal of Intelligent Material Systems and Structures 6, 307 (1995); Woojin Lee, “Polymer Gel Based Actuator: Dynamic model of gel for real time control”, PhD Thesis, MIT (1996); and Kato, N., Takizawa, Y., Takahashi, F., “Magnetically Driven Chemomechanical Device with Poly(N-Isopropylacrylamide) Hydrogel Containing Gamma-Fe2O3”, Journal Of Intelligent Material Systems And Structures 8, 588 (1997).
In ionic polymers, the electrostatic repulsion between similar charges can considerably increase the swelling forces. The amount of repulsion depends on the number of charges and on the concentration of counter-ions present in the polymer. The increase in counter-ion concentration causes an increase of the charge shielding, this resulting in reduction of the repulsion forces. This phenomenon can also be envisaged as an osmotic pressure effect. The concentration of electric charges within the crosslinked polymer is higher than in the solvent. Therefore, the solvent enters the polymer in an attempt to equalize the osmotic pressure inside and outside the hydrogel.
For background information on pH-sensitive hydrogels see the articles by S. K. De et al., “Equilibrium Swelling and Kinetics of pH-responsive Hydrogels: Models, Experiments, and Simulations”, Journal of Microelectromechanical Systems, 11, 544 (2002) and “A chemo-electro-mechanical mathematical model for simulation of pH sensitive hydrogels”, Mechanics of Materials 36, 395 (2004); H. Li, T. Y. Ng, Y. K. Yew, K. Y. Lam, “Modeling and simulation of the swelling behavior of pH-stimulus-responsive hydrogels”, Biomacromolecules 6, 109 (2005); Marra, S. P., Ramesh, K. T., Douglas, A. S., “Mechanical Characterization of Active Poly(vinyl alcohol)-Poly(acrylic acid) Gel”, Materials Science and Engineering C 14, 25 (2001); Fei, J. Q., Gu, L. X., “PVA/PAA Thermo-crosslinking Hydrogel Fiber: Preparation and pH-sensitive Properties in Electrolyte Solution”, European Polymer Journal 38, 1653 (2002); and Johnson, B., Niedermaier, D. J., Crone, W. C., Moorthy, J., Beebe, D. J., “Mechanical Properties of a pH Sensitive Hydrogel”, Proceedings of the Annual Conference of the Society for Experimental Mechanics, Milwaukee, EUA, 2002; Marra, S. P., Ramesh, K. T., Douglas, A. S., “Mechanical Characterization of Active Poly(vinyl alcohol)-Poly(acrylic acid) Gel”, Materials Science and Engineering C 14, 25 (2001).
U.S. Pat. No. 5,015,843 teaches a chemical sensor that can detect the presence of a chemical species in solution based on the swelling of a polymer upon reaction to the chemical species. The polymer swelling is indirectly determined by measuring the light reflected from a reflector directly or indirectly attached to the polymer. In this way, an increase or reduction of the polymer size changes the distance between the reflector and a source of light. Measurements of the intensity of light reflected by the reflector indicate the amount of swelling or shrinking of the polymer in response to the chemical species. However this U.S. patent does not aim to measure pH nor it makes use of a fiber Bragg grating as in the present invention.
In chemical processes pH is the most widely used chemical parameter and provides information on the concentration of hydrogen and hydroxyl ions in an aqueous solution. pH measures the ionic activity of hydrogen ions in solution. The concentration and ionic activity are related, and are the same for ionic solutions for which the dilution can be considered as infinite.
State-of-the-art pH measurements include pH indicators using dyes, the color of which is pH-dependent and the pH electrode, the working principle of which is the electrochemical cell. Non-conventional techniques include the ISFET (Ionic Selective Field Effect Transistor) and the optical sensors. In an optical sensor, the pH measurement is carried out by the modulation of the light intensity through absorption, reflection or fluorescence of hydrogen ion-sensitive chromophores.
On the other hand, certain drawbacks in the detection of metallic corrosion in difficult access equipment and installations have not yet been solved in the several industrial fields.
Sensors for evaluating corrosion are normally based on sacrificial bodies or on the monitoring of the cathodic reaction occurring concomitant to the oxidation process and corrosion reduction.
In the petroleum industry, metallic corrosion can cause the premature failure of equipments as well as oil leaks, leading to human and environment safety risk and with expensive maintenance operations. Real-time, dynamic knowledge of the conditions that generate metallic corrosion can be a very precious tool for the prevention and control of its consequences.
Much of the research on metallic corrosion applied to the petroleum industry is focused on the determination of the medium corrosiveness for the evaluation of material corrosion rates. The corrosion rate of most of metals is affected by pH. In general, the effect of pH on the corrosion rate follows three standards. According to the first of these standards, applicable to metals that are soluble in acid, such as iron, the corrosion rate is fairly high for pH<4, while for 4<pH<10, it depends on the concentration of oxidants. The second standard applies to metals such as aluminum and zinc, the corrosion rate of which is high for pH<4 and pH>8. Finally, noble metals, such as gold and platinum, are not affected by pH.
The correlations used for estimating pH in subsurface environments, based on measurements carried out at the surface, came out to be fairly imprecise. A sensor that could be installed downhole and in other sites of the oil production system would constitute a useful tool to check the correlations and would help the selection of the most suitable materials to be used in equipment and pipes.
The possibility to monitor, continuously and permanently, downhole parameters such as temperature (T), pressure (P) and flow rate in the petroleum wells is at the origin of what is presently called intelligent or smart wells.
Nowadays, sensors for downhole pressure, temperature and flow rate based on the FBG technique are operative. On this respect see the articles by A. D. Kersey, “Optical fiber sensors for permanent downwell monitoring applications in the oil and gas industry”, IEICE Transactions on Electronics E83C, 400 (2000); and Ph. M. Nellen, P. Mauron, A. Frank, U. Sennhauser, K. Bohnert, P. Pequignot, P. Bodor, H. Brandle, “Reliability of fiber Bragg grating based sensors for downhole applications”, Sensors Actuators A-Physical 103, 364 (2003).
However, there are still important downhole variables to be monitored, one of these being pH. Real-time pH measurement, together with pressure, temperature and flow rate, can provide a better understanding of the corrosion process and be useful to infer corrosion rate in oil plants.
Thus, it is clear that the technique still needs an optical fiber pH sensor where the volume change of a pH-sensitive hydrogel is transmitted to an optical fiber containing a Bragg grating, the induced strain variation changing the wavelength of the Bragg grating, yielding a pH sensor, such sensor being described and claimed in the present application.